The goal of the proposed research is to develop a theory of how people use knowledge of language to comprehend sentences. The research examines the cognitive processes of normal, intact adults during reading and listening. The proposed experiments are designed to uncover the principles that govern readers' and listeners' use of grammatical information in sentence parsing (i.e. the assignment of structure to a sentence, needed to extract meaning), and is predicated on the assumption that these principles will form a distinct subset of the principles that govern cognition in general. The proposal has three distinct foci. One series of experiments examines the basic assumptions of the 'depth-first' theory that has motivated our previous research. Our theoretical position has been challenged by claims that parsing is breadth-first and parallel, so that multiple alternative structural analyses are activated. We propose strong tests of these two positions. A second series of experiments tests a claim we have made, that two different types of phrases are processed quite differently. Primary relations are processed according to the depth-first principles we have advanced earlier, being attached into a single determinate syntactic structure. Non-primary relations are instead associated with a thematically-defined domain of a sentence, and semantic and non-linguistic sources of information can be used, following a process we call construal, to determine their specific interpretation within this domain. A third series of experiments examines the processing of sentences with long-distance dependencies. It tests some current theoretical claims about how the moved element (the filler) can be related to the position in which it is interpreted (the gap), and advances some novel ideas about how fillers might be interpreted by receiving a role in event structure or by receiving a discourse-linked interpretation. The proposed research uses eyetracking and other techniques of experimental cognitive psychology, and is designed to the development of psycholinguistic theory. Recent advances in psycholinguistic theory have had substantial impact on the analysis of language disorders, especially aphasia. We anticipate that these new analyses will lead to increased understanding and improved treatment of such disorders.